The present invention relates to a method of measuring the density of standing plants, in order to control the rate of operation of harvest machines, by use of an ultrasonic field which permeates the uncut growth immediately forward of the cutters, the drop in intensity of which between the untrasonic transmitter and receiver is converted into a control signal. The invention relates furthermore to apparatus on a harvest machine such as a combined harvester for controlling the machine's operating rate by measurement of the density of growth using an ultrasonic field immediately forward of the harvest tools such as cutting knives in the direction of travel, said apparatus including an ultrasonic transmitter and an ultrasonic receiver arranged at right angles to that direction on opposite sides of the machine.
It is known, when measuring the density of grain crop growth, to set cut plants in a strip of predetermined width and to subject this as a medium of irregular distribution in a homogeneous medium air to an ultrasonic field, the effect upon which is used as a measure of the average distribution. It is also known to increase the width of the sounded strip to that of the operating width of the machine, making it possible to obtain automatically the growth density and density changes in front of the harvest machine cutters, so that the rate of the harvest machine can be controlled in dependence upon the measured plant density value. For this purpose, a method has already been developed for ultrasonically measuring the growth density or changes in the growth density sufficiently in advance of the harvester cutters, without alteration to the plants. To do this, the necessary ultrasonic transmitter and receiver are arranged on a suitable carrier in the immediate vicinity of the cutters so that the data obtained can be evaluated and used at once to control, for instance, the forward rate of travel of the harvester.
It has already been realized that suitable values for measuring the density of plant growth by means of ultrasonic fields on harvest machines can only be provided when interference noise, especially that made by the harvest machine itself, is suppressed. For this reason, filter means have been introduced into the electro-acoustic transformer to hold back the interference frequencies.
The precautions taken in the past, however, have been insufficient because the frequency spectrum of interference noise produced by the multi-blade cutters extends high into the ultrasonic zone, frequently causing incorrect measurements on the known types of ultrasonic measuring devices for harvest machines. With the known methods and devices, the intensity of the ultrasonic vibrations is tested only after passing through the strip being measured, and the amplitude of the acoustic pressure is used to form the control signal. The heterodyne interference in the ultrasonic field of measurement can lead to considerable distortion of the amplitude, whereby even an increase in the amplitude is possible although a decrease in the intensity of the ultrasonic vibrations would normally be expected due to absorption and reflection over a large distance.
Furthermore, the strip being ultrasonically measured can only be a short distance in front of the cutting means and thus in a zone in which the level of interference is unavoidably high. Apart from this, the absorption alone is not proportional to the density of plant growth because of weeds, undergrowth, and bald or dense patches. In all, on large harvest machines, the signal reaching the receiver is so weak and distorted that on the one hand it is dependent on the contingencies in the vicinity of the receiver, and on the other hand, even the normal differences in growth across the cutting width have little in common with the true average value.
It is therefore the purpose of the present invention, while avoiding the above-mentioned disadvantages, to provide a control signal which is sufficiently proportional to the growth density across the entire width of the cutters on a large harvest machine, and which is independent of interference at ultrasonic frequencies produced by the cutting means in the harvester.